In recent years, the effect of exposure to light such as sunlight irradiation on the skin is becoming a serious worldwide problem. In particular, in the United States, Europe, and Australia, the increase in incidence of skin cancer has become a serious problem. One of the factors of the increase in skin cancer is an ongoing increase in the amount of exposure to ultraviolet light in daily life due to the destruction of the ozone layer by chlorofluorocarbon. In today's world, it is impossible to avoid the risk of forming abnormal skin cells caused by exposure to light, specifically, the risk of photocarcinogenesis (i.e., skin cancer induced by exposure to light, such as ultraviolet light).
Conventionally, surgical treatment, chemotherapy, and radiation therapy are combined for the treatment of skin cancer. However, depending on the detection timing and the degree of symptoms of skin cancer, it is common that skin cancer may not be effectively treated, and that the treatment may create a considerable burden on daily life. These problems significantly degrade the quality of life (QOL) of individuals. Therefore, there has been a strong demand for the establishment of a method for preventing the occurrence of skin cancer.
Conventionally, the prevention of skin cancer caused by ultraviolet light is carried out by a known method in which the skin is protected from ultraviolet light using an externally applied drug containing an ultraviolet light-absorbing agent such as octyl methoxycinnamate, butyl methoxydibenzoylmethane, and benzophenone; and an ultraviolet light-scattering agent such as titanium oxide and zinc oxide.
However, while ultraviolet light-absorbing agents and ultraviolet light-scattering agents prevent exposure to ultraviolet light, these agents have been reported to have adverse effects, such as skin irritation. In other words, while these agents inhibit the formation of abnormal skin cells caused by exposure to light, these agents have the potential to cause disorders due to the contact stimulus and the like.
Further, such drugs containing the above ingredients must be applied to the skin before exposure to ultraviolet light, because the purpose is to prevent exposure of the skin to ultraviolet light. Accordingly, in the case of ultraviolet light-absorbing agents and ultraviolet light-scattering agents, the skin cannot be prevented from exposure to ultraviolet light if the user forgets to apply these agents to the skin or when these agents are removed from the skin by perspiration or the like, leading to induction of DNA damage. As a result, the risk of forming abnormal skin cells caused by exposure to light cannot be avoided.
Accordingly, if it is possible to inhibit the formation of abnormal cells in the skin tissue by an ingredient that does not adversely affect the skin, even when the skin is directly exposed to sunlight such as ultraviolet, it will be an effective preventive measure against the formation of abnormal skin cells, which cannot be achieved with ultraviolet light-absorbing agents or ultraviolet light-scattering agents. Under such circumstances, there is a strong demand for the development of a preventive agent capable of suppressing the formation of abnormal skin cells caused by exposure to light in the skin tissue, without causing skin irritation and the like.
Meanwhile, purine nucleic acids such as adenosine phosphates have the following effects: moisture retention and prevention or amelioration of wrinkles (see Patent Literature 1); prevention or amelioration of pigmentation (see Patent Literature 2); promotion of collagen production (see Patent Literature 3); and the like, and have been drawing attention as ingredients having cosmetically and pharmaceutically beneficial effects on the skin.
Further, there are known documents, regarding the anti-cancer effect of the purine nucleic acids, in which the suppression of growth of cancer cells induced by chemical substances is investigated (see Non-Patent Literature 1 and 2). However, these investigations did not actually confirm the effect of the purine nucleic acids on cancer cells caused by sunlight such as ultraviolet. The effect was merely investigated by administration into the abdominal cavity, which is not easily available to the user.
As described above, no investigation has been made on the relationship between photocarcinogenesis and external (dermal) application of the purine nucleic acids, and it is completely unknown how the purine nucleic acids affect the suppression of formation of abnormal skin cells caused by exposure to light, or the prevention or treatment of photocarcinogenesis when applied externally.